1 /* 2 * parse_vdso.c: Linux reference vDSO parser 3 * Written by Andrew Lutomirski, 2011-2014. 4 * 5 * This code is meant to be linked in to various programs that run on Linux. 6 * As such, it is available with as few restrictions as possible. This file 7 * is licensed under the Creative Commons Zero License, version 1.0, 8 * available at http://creativecommons.org/publicdomain/zero/1.0/legalcode 9 * 10 * The vDSO is a regular ELF DSO that the kernel maps into user space when 11 * it starts a program. It works equally well in statically and dynamically 12 * linked binaries. 13 * 14 * This code is tested on x86. In principle it should work on any 15 * architecture that has a vDSO. 16 */ 17 18 #include <stdbool.h> 19 #include <stdint.h> 20 #include <string.h> 21 #include <limits.h> 22 #include <elf.h> 23 24 /* 25 * To use this vDSO parser, first call one of the vdso_init_* functions. 26 * If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR 27 * to vdso_init_from_sysinfo_ehdr. Otherwise pass auxv to vdso_init_from_auxv. 28 * Then call vdso_sym for each symbol you want. For example, to look up 29 * gettimeofday on x86_64, use: 30 * 31 * <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday"); 32 * or 33 * <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday"); 34 * 35 * vdso_sym will return 0 if the symbol doesn't exist or if the init function 36 * failed or was not called. vdso_sym is a little slow, so its return value 37 * should be cached. 38 * 39 * vdso_sym is threadsafe; the init functions are not. 40 * 41 * These are the prototypes: 42 */ 43 extern void vdso_init_from_auxv(void *auxv); 44 extern void vdso_init_from_sysinfo_ehdr(uintptr_t base); 45 extern void *vdso_sym(const char *version, const char *name); 46 47 48 /* And here's the code. */ 49 #ifndef ELF_BITS 50 # if ULONG_MAX > 0xffffffffUL 51 # define ELF_BITS 64 52 # else 53 # define ELF_BITS 32 54 # endif 55 #endif 56 57 #define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x 58 #define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x) 59 #define ELF(x) ELF_BITS_XFORM(ELF_BITS, x) 60 61 static struct vdso_info 62 { 63 bool valid; 64 65 /* Load information */ 66 uintptr_t load_addr; 67 uintptr_t load_offset; /* load_addr - recorded vaddr */ 68 69 /* Symbol table */ 70 ELF(Sym) *symtab; 71 const char *symstrings; 72 ELF(Word) *bucket, *chain; 73 ELF(Word) nbucket, nchain; 74 75 /* Version table */ 76 ELF(Versym) *versym; 77 ELF(Verdef) *verdef; 78 } vdso_info; 79 80 /* Straight from the ELF specification. */ 81 static unsigned long elf_hash(const char *s_name) 82 { 83 unsigned long h = 0, g; 84 const unsigned char *name = (const unsigned char *)s_name; 85 86 while (*name) 87 { 88 h = (h << 4) + *name++; 89 g = h & 0xf0000000; 90 if (g) 91 h ^= g >> 24; 92 h &= ~g; 93 } 94 return h; 95 } 96 97 void vdso_init_from_sysinfo_ehdr(uintptr_t base) 98 { 99 size_t i; 100 bool found_vaddr = false; 101 102 vdso_info.valid = false; 103 104 vdso_info.load_addr = base; 105 106 ELF(Ehdr) *hdr = (ELF(Ehdr)*)base; 107 if (hdr->e_ident[EI_CLASS] != 108 (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) { 109 return; /* Wrong ELF class -- check ELF_BITS */ 110 } 111 112 ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff); 113 ELF(Dyn) *dyn = 0; 114 115 /* 116 * We need two things from the segment table: the load offset 117 * and the dynamic table. 118 */ 119 for (i = 0; i < hdr->e_phnum; i++) 120 { 121 if (pt[i].p_type == PT_LOAD && !found_vaddr) { 122 found_vaddr = true; 123 vdso_info.load_offset = base 124 + (uintptr_t)pt[i].p_offset 125 - (uintptr_t)pt[i].p_vaddr; 126 } else if (pt[i].p_type == PT_DYNAMIC) { 127 dyn = (ELF(Dyn)*)(base + pt[i].p_offset); 128 } 129 } 130 131 if (!found_vaddr || !dyn) 132 return; /* Failed */ 133 134 /* 135 * Fish out the useful bits of the dynamic table. 136 */ 137 ELF(Word) *hash = 0; 138 vdso_info.symstrings = 0; 139 vdso_info.symtab = 0; 140 vdso_info.versym = 0; 141 vdso_info.verdef = 0; 142 for (i = 0; dyn[i].d_tag != DT_NULL; i++) { 143 switch (dyn[i].d_tag) { 144 case DT_STRTAB: 145 vdso_info.symstrings = (const char *) 146 ((uintptr_t)dyn[i].d_un.d_ptr 147 + vdso_info.load_offset); 148 break; 149 case DT_SYMTAB: 150 vdso_info.symtab = (ELF(Sym) *) 151 ((uintptr_t)dyn[i].d_un.d_ptr 152 + vdso_info.load_offset); 153 break; 154 case DT_HASH: 155 hash = (ELF(Word) *) 156 ((uintptr_t)dyn[i].d_un.d_ptr 157 + vdso_info.load_offset); 158 break; 159 case DT_VERSYM: 160 vdso_info.versym = (ELF(Versym) *) 161 ((uintptr_t)dyn[i].d_un.d_ptr 162 + vdso_info.load_offset); 163 break; 164 case DT_VERDEF: 165 vdso_info.verdef = (ELF(Verdef) *) 166 ((uintptr_t)dyn[i].d_un.d_ptr 167 + vdso_info.load_offset); 168 break; 169 } 170 } 171 if (!vdso_info.symstrings || !vdso_info.symtab || !hash) 172 return; /* Failed */ 173 174 if (!vdso_info.verdef) 175 vdso_info.versym = 0; 176 177 /* Parse the hash table header. */ 178 vdso_info.nbucket = hash[0]; 179 vdso_info.nchain = hash[1]; 180 vdso_info.bucket = &hash[2]; 181 vdso_info.chain = &hash[vdso_info.nbucket + 2]; 182 183 /* That's all we need. */ 184 vdso_info.valid = true; 185 } 186 187 static bool vdso_match_version(ELF(Versym) ver, 188 const char *name, ELF(Word) hash) 189 { 190 /* 191 * This is a helper function to check if the version indexed by 192 * ver matches name (which hashes to hash). 193 * 194 * The version definition table is a mess, and I don't know how 195 * to do this in better than linear time without allocating memory 196 * to build an index. I also don't know why the table has 197 * variable size entries in the first place. 198 * 199 * For added fun, I can't find a comprehensible specification of how 200 * to parse all the weird flags in the table. 201 * 202 * So I just parse the whole table every time. 203 */ 204 205 /* First step: find the version definition */ 206 ver &= 0x7fff; /* Apparently bit 15 means "hidden" */ 207 ELF(Verdef) *def = vdso_info.verdef; 208 while(true) { 209 if ((def->vd_flags & VER_FLG_BASE) == 0 210 && (def->vd_ndx & 0x7fff) == ver) 211 break; 212 213 if (def->vd_next == 0) 214 return false; /* No definition. */ 215 216 def = (ELF(Verdef) *)((char *)def + def->vd_next); 217 } 218 219 /* Now figure out whether it matches. */ 220 ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux); 221 return def->vd_hash == hash 222 && !strcmp(name, vdso_info.symstrings + aux->vda_name); 223 } 224 225 void *vdso_sym(const char *version, const char *name) 226 { 227 unsigned long ver_hash; 228 if (!vdso_info.valid) 229 return 0; 230 231 ver_hash = elf_hash(version); 232 ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket]; 233 234 for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) { 235 ELF(Sym) *sym = &vdso_info.symtab[chain]; 236 237 /* Check for a defined global or weak function w/ right name. */ 238 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC) 239 continue; 240 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL && 241 ELF64_ST_BIND(sym->st_info) != STB_WEAK) 242 continue; 243 if (sym->st_shndx == SHN_UNDEF) 244 continue; 245 if (strcmp(name, vdso_info.symstrings + sym->st_name)) 246 continue; 247 248 /* Check symbol version. */ 249 if (vdso_info.versym 250 && !vdso_match_version(vdso_info.versym[chain], 251 version, ver_hash)) 252 continue; 253 254 return (void *)(vdso_info.load_offset + sym->st_value); 255 } 256 257 return 0; 258 } 259 260 void vdso_init_from_auxv(void *auxv) 261 { 262 ELF(auxv_t) *elf_auxv = auxv; 263 for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++) 264 { 265 if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) { 266 vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val); 267 return; 268 } 269 } 270 271 vdso_info.valid = false; 272 } 273
